Next, follow some of the examples below to set up your mail configuration. For
extensive documentation of other email solutions, including examples of
integration with most common SMTP servers, libraries, and frameworks, see
Mailjet's documentation.

Debian

CentOS

Modify the Postfix configuration options. Postfix configuration options are
set in the main.cf file. Open the file with the text editor of your
choice:

user@test-instance:~# sudo vi /etc/postfix/main.cf

If they exist, comment out the following lines:

# default_transport = error
# relay_transport = error

Add the Mailjet SMTP service by adding the following line to the end of the
file:

relayhost = in.mailjet.com:2525

Note: You must use port 2525, as port 25 is not allowed on
Compute Engine.

Next, add the following lines to enforce SSL/TLS support and to configure
STMP authentication for these requests. A simple access and security layer
(SASL) module handles authentication in the Postfix configuration. Add
the following lines to the end of the file: